In the nuclear power industry, it is important to ascertain that safety related motor-operated valves ("MOV's"), are set and maintained correctly to operate over the full range of expected normal and abnormal events. MOV's each typically include a valve and a motor operator coupled with the valve through a stem of the valve.
Many plants currently use systems that measure displacement of a spring pack associated with the motor actuator as an indicator of forces in the valve system. The user first calibrates the system by backseating the valve against a calibrated load cell, while recording the outputs from both a spring pack displacement sensor and the load cell. The resulting calibration is thereafter used to infer stem loads during seating, unseating and backseating from traces of spring pack displacement alone.
This apparently straightforward method though is subject to many problems. One is that spring packs typically have substantial initial compression. This means that the valve stem force must build up to a significant level before any further spring pack compression can take place. As a consequence there is a large dead zone within which no forces can be measured. Conversely, there may be clearance between the spring pack and spring pack cavity, allowing the spring pack to displace that amount of clearance in response to virtually no stem force. Even when set with no initial compression and no initial clearance, the compression of the spring pack tends to be on-linear with respect to the force. Furthermore, a condition of grease buildup in the spring pack is not uncommon, and this condition can greatly limit spring pack compression even under the application of very great force. Finally, gear friction forces can also cause the spring pack to compress and this compression can be misread as being due to stem force. In summary, making stem force measurements from spring pack displacement traces is difficult under the best of circumstances, and even then it may require a great deal of art.
Much useful information could be obtained from the motor current trace. This is easier to obtain than spring pack displacement. Motor current, since it responds to all load demands on the motor, can be used to infer stem loads during seating and unseating, packing forces and other system friction forces, and variations in these forces over time. The main difficulty is quantifying these forces, and separating one from another.
The most desirable measurement for accurately monitoring the dynamic events within the MOV would be the direct measure of valve stem load through the use of strain gauges attached to the stem. However, it is impractical to retrofit existing valves in this manner. For most valves, a stem mounted sensor is usable only over a small part of the stem stroke and must be removed to permit full operation of the valve.
Applicant has invented an MOV valve yoke strain measurement system as an acceptable alternative approach to continuous, indirect monitoring of dynamic stem loads. Since valve stem forces cause equal and opposite yoke reaction forces, the resulting yoke strain is an accurate indicator of stem force over the entire valve stroke. The new yoke strain sensor is the subject of related co-pending applications Ser. No. 185,210, filed Apr. 22, 1988 and Ser. No. 87,541, filed Aug. 20, 1987, now U.S. Pat. No. 4,805,451 before both incorporated herein in their entirety, by reference.
The yoke strain sensor is easily applied and calibrated and, since it resists the harsh environments of nuclear power plants, it can be left permanently in place for repeated surveillance tests and for continuous monitoring. The installed yoke strain sensor need be calibrated to the MOV only once. Calibration involves relating yoke strain with valve stem force and is just a function of the particular yoke geometry and the location of the yoke strain sensor on the yoke. After calibration, the yoke strain sensor can be interchanged with any other yoke strain sensor of like strain sensitivity without changing that calibration.
Calibration of the yoke strain sensor to the MOV can be accomplished in different ways. One way is through measurement of the strain that results in the valve stem when the valve is seated (or backseated). The calibration load need not be known. Simple geometrical relationships exist between strain in the valve stem and axial forces or stresses on the valve stem for both unthreaded and threaded circular cylinders (i.e. valve stems). The calibration load may be deduced from measured valve stem strains using these relationships.
One problem in measuring the strain to which the MOV valve stem are subjected is that typically only a portion of the valve stem is exposed and may not be easily accessible through the yoke. Moreover, the attachment of conventional axial (longitudinal) strain gauges directly to valve stems is tedious and time consuming at best, and difficult or virtually impossible for some MOV geometries. In some instances, the length of the valve stem externally accessible may be less than the length needed for installing a conventional axial strain measuring device. Axial strain measuring devices are also subject to error induced by bending or buckling of the valve stem between the axially spaced attachment points of such devices.
It is possible to determine axial strain (and stress) in generally cylindrical members from diametral strain. The ratio of unit diametral contraction to unit axial elongation for a body, referred to as the Poisson's ratio, is known or may otherwise be independently determined for different alloys, dimensions and configurations (hollow/solid, threaded/unthreaded, etc.) typically employed in the MOV valve stems found in nuclear power plants. Typically, the Poisson's ratio for valve stems encountered in power plant MOV's range from about 0.2 to 0.4. Thus, by measuring the diametral deformation on the valve stem, the axial strain and thus the axial force on the valve stem can be determined.